the other, (harder) variant on this problem is if you know that the fake coin has a different weight than the others, but not if it's heavier or lighter...
Incidentally, the first time I heard this problem (or a variant) was in Piers Anthony's "With a Tangled Skein", when the protagonist has to pick out the good soul from a bunch of demons, using some sort of "good-vs-evil" balance, in only two weighings. (I think it was in Skein, anyway; it was definitely in one of the Incarnations of Immortality books. Actually, I am really unsure of the details, so if anyone wants to correct me, please do.)
The next time I heard it was while doing an internship at Microsoft; apparently it's a popular puzzle to ask during interviews out there..